The present invention relates to a method of making hollow articles and more particularly to a method of manufacturing a section of a hollow aircraft rotor blade such as used on a circulation control blade for a helicopter.
In a circulation control rotor (CCR) helicopter, the blades are rigidly fixed to the hub and lift is controlled by controlling the circulation of air around each blade. The primary advantage of a CCR helicopter is the reduction of weight of the entire rotor and reduction in number of moving parts associated with the blades; both of these result from the fact that the lift of each blade is modified by changing the circulation of air around it rather than by changing its angle of attack. The resulting rotor is lighter in weight thereby allowing a greater payload for the craft, and also is more reliable.
Circulation control airfoils utilize jets of air blown tangentially to the upper surface of the trailing edge of the airfoil to generate high lift coefficients. Generally the chordwise position of the blowing slot is determined prior to construction so as to inject the jet at the most beneficial chordwise position. The chordwise position is determined by calculating theoretical pressure distributions for the angles of attack and lift coefficients the airfoil is expected to experience. The jet should be located prior to the aft pressure peak so as to maintain airflow attachment on the airfoil. By increasing the circulation about the airfoil, high lift coefficients can be achieved.
The blades of a circulation control rotor must have an air passageway therethrough and thus are hollow. Many CCR blades are made of metal, such as aluminum, and are manufactured in the following manner. An aluminum alloy ingot is placed into a press chamber and is heated up to the plastic state. The press chamber is provided with a cavity corresponding to the shape of the outer surface of the spar with predetermined allowances for subsequent machining, as well as with a mandrel having a cross-sectional shape corresponding to the cross-sectional shape of the spar cavity. The mandrel and the cavity are disposed relative to each other so that the space defined therebetween corresponds to the spar wall's thickness with an allowance for machining. The blank portions, which are sectioned by the mandrel partitions, are pressed over the inner surface of the spar, whereafter they are welded under pressure and heat during the shaping of the outer surface in the press chamber cavity.
The resulting blank comprises a hollow press shaped piece of a constant cross-sectional shape. In order to obtain a spar from this piece, the latter is subjected to a number of production treatment steps including heat treatment, dressing, machining (milling, grinding, scraping, polishing), as well as twisting about the longitudinal axis, surface hardening application of electrolytic coating. These steps are required to obtain sufficient mechanical properties and dynamic strength, a predetermined aerofoil shape, as well as corrosion resistance.
The above-mentioned production treatment steps require complicated, cumbersome and expensive equipment and a large volume of wastes results from this treatment. In addition, a spar is obtained with a constant cross-sectional shape of the cavity over the entire length thereof and generally it is more desirable to have a variable cross-sectional shape over its length.
One such attempt to provide an improvement over a metal blade having a constant cross-sectional shape of the cavity is shown and described in U.S. Pat. No. 3,967,996, which issued July 6, 1976, to Nikolai Kamov et al. In this patent, a method of manufacturing hollow pieces is described which comprises the steps of forming a blank of single flexible sheets each having a layer of a gluing substance applied thereto, said sheets being placed on a mandrel and subjected to a preliminary compression during which the agglutination of the sheets takes place so as to impart to the blank a stiffness and to shape it into an unclosed profile as required for subsequent steps, bringing the ends of the blank together so as to define a cavity of a closed profile, accommodating an elastic bag in said cavity and placing the blank into a mould having the inner surface corresponding to the outer surface of the piece with subsequent feeding of pressure into the elastic bag for a final pressing during which the hollow piece of a predetermined shape is obtained.